What does "Mesodermal cell from a single cell during the growth of embryo" and "coelom formed from pouches pinched off endoderm" mean???

Mesodermal cell from a single cell during the growth of embryo mean:

A single cell (zygote) undergo meiosis and differentiation to develop into embryo. In this embryo during gastrulation, only some cell migrate inward contributing to mesoderm between the two germ layer i.e. ectoderm and endoderm. 

Coelom formed from pouches pinched off endoderm mean:

When the embryo reaches the gastrulation stage. At this point, there are two layers of cells - the ectoderm (outermost) and the endoderm (innermost) layers. The mesoderm (middle layer) begins to form as two "pockets" of tissue (one above the endoderm, and one below) are formed via folding of the endoderm. These "pockets" begin to grow larger, and as they do so, they extend towards each other. When the two "pockets" of cells meet, the mesoderm is formed - a complete layer of tissue right in between the endoderm and ectoderm layers. This then leads to the formation of a coelom.

 

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